Following exposure to CCl4, rat hepatic mitochondria show a progressive loss, then recovery, of oxidative phosphorylation. Paralleling this aspect of damage is a loss of the mitochondrial intramembranal enzyme sulfite oxidase. The loss appears to be the result of an increase in the permeability of the outer mitochondrial membrane as a result of CCl4 intoxication. The recovery of normal levels of sulfite oxidase follows the recovery of respiratory control, and the restoration of outer membrane integrity. Associated with the recovery process is an increased contamination of the mitochondrial fraction with membrane fragments that by morphological examination and biochemical assay seem to be derived from the ER of the damaged cell. The purpose of the proposed research is to study the mechanism of synthesis and translocation of sulfite oxidase that affects its replacement, apparently in the face of normal outer membrane integrity. Further, we seek to ascertain what the presence of the microsomal contamination represents with regard to the restoration of mitochondrial function, and the recovery of normal levels of sulfite oxidase. The molybdenum-containing sulfite oxidase, which couples sulfite oxidation to phosphorylation of ADP at complex IV of the mitochondrion, is a useful tool for a study of enzyme synthesis and translocation. Transport of the enzyme will be demonstrated by the coupling of sulfite oxidation to ADP phosphorylation and uptake of oxygen. To determine if vectorial discharge of the enzyme into the mitochondrion occurs during synthesis, the sensitivity of the enzyme to limited proteolysis will be tested. The results of this study will furnish information relating to mitochondrial biogenesis, the response of mammalian cells to chemical damage, and those processes that direct a newly synthesized protein to its functional location.